1. Field of the Invention
The present invention relates to a transmitter or a transmitter part of a transceiver and more particularly to the power control of a transmitter power amplifier.
2. Description of the Prior Art
Recent developments in cellular telephones has stimulated interest in power control of a transmitter power amplifier because in an attempt to reduce interference and economise on battery power, output powers are regulated to be sufficient for a receiving station to obtain an acceptable signal. A typical transmitter power amplifier control circuit is disclosed in British Patent Specification GB-A-2 220 808 and comprises a power amplifier having an input, an output and a gain control input, a directional coupler couples out a portion of the amplifier's output signal and applies it generally as a dc representation of the output to one input of a comparator to a second input of which is applied a locally generated reference power waveform, an output from the comparator is applied to the gain control input of the power amplifier. The gain control input normally controls the supply voltage of the power amplifier. This type of power amplifier control is normally applied because the amplifier architecture does not normally have overall feedback. In fact in the interests of amplifier stability, feedback is avoided in general practice because the frequencies are high.
In certain applications, for example TDMA digital trunking systems, a linear power amplifier with a good overall efficiency is required. Typically in private mobile radio non-linear (class C) amplifiers are used which are unsuitable for digital trunking systems. A known type of linear power amplifier architecture is a Cartesian loop amplifier in which an input signal is represented by orthogonal vectors I (in-phase) and Q (quadrature phase). Another known type of linear power amplifier is a polar loop amplifier.